Orthogonal frequency division multiplexed (OFDM) communications systems use a digital multi-carrier modulation scheme in which symbol components are transmitted over multiple frequency-spaced subcarriers using a relatively low symbol rate. Among potential advantages of wireless OFDM communications systems are improved resistance to narrow-band interference and frequency-selective fading. Conventional wireless OFDM communications systems include WiFi wireless local area networks conforming to the IEEE 802.11(g) standards and WiMAX metropolitan area networks (MANs) conforming to the IEEE 802.16 standards. Wireless MANs, such as some WiMAX MANs, may support mobile terminal operations using a cellular architecture employing multiple distributed base stations.
Recent years have seen the migration of telephony services from traditional circuit-switched approaches to packet-switched approaches that convey voice and other information over the internet. For example, VoIP (voice over IP) protocols may use a broadband internet connection to provide telephony services substantially similar to services provided by traditional circuit-switched telephony networks. Wireless networks that support IP transport, such WiMAX networks, may be used for VoIP and similar telephony applications.
Among the services supported by conventional telephone networks is location determination in support of emergency response. For example, in an “E911” service, dialing “911” at traditional telephone in most of the United States and Canada will link the caller to an emergency dispatch center, which receives information identifying the location for the origin of the call from the telephone network. Cellular telephone system operators are migrating their networks to provide a similar locating capability using, for example, GPS (global positioning system) chips located in terminals and/or triangulation using multiple base stations. If 911 is dialed from a VoIP service, the call may not go anywhere at all, or it may go to a location that is not particularly near the caller. In May 2005 the FCC issued an order requiring VoIP providers to offer 911 service to all their subscribers, but some VoIP providers have yet to meet these requirements.